The present invention relates to a tire pressure monitoring system to be mounted, for example, on a vehicle, and particularly to a technique for accurately monitoring tire pressure with a relatively simple structure.
Drivability or maneuvering stability of vehicles greatly depends on air pressure of tires as contacting points with the road surface as well as the structure of suspensions or a steering device. For example, a tire with insufficient air pressure inevitably decreases its rigidity and increases its contact area with the road surface. By this and also due to lack of uniformity (lack of uniformity of the shape), the steering response deteriorates to a greater extent. Because decreased tire pressure results in an increase in temperature of the tire or an occurrence of standing wave during high speed driving of the vehicle as well as deteriorated fuel economy, it is therefore necessary to strictly prevent the decrease in the tire pressure in terms of stability and an economical viewpoint. Conventionally, drivers or driving administrators usually check and maintain the tire pressure by means of an air charger (air gauge) equipped in gas stations and the like.
Air pressure lowers due to a leakage of air, which possibly happens if a nail or pieces of broken glass cut the tire during the running of the vehicle and thereby creating a hole in the tread or if the air tight seal between the tire bead and the wheel rim is insufficient. However, slow leak wherein air leaks gradually and very slowly has become common in recent tubeless tires. In the case of slow leak, it is difficult for the driver to notice lowering of the air pressure (underinflation of tires) at an early stage of the driving because the drivability or maneuvering stability deteriorates very slowly.
For this reason, as a tire pressure monitoring system (TPMS) for monitoring lowering of air pressure during the driving of a vehicle, an air pressure sensor including a distortion gauge, a transmitter, a battery, etc. is mounted on each wheel (hub). This kind of tire pressure monitoring system is complicated in its structure and the usage thereof is strict, which may cause a malfunction of the air pressure sensor during the driving on a rough road or the like. Further, monitoring the air pressure can not be performed if the battery is run out.
Therefore, in recent years, the tire pressure monitoring system has mainly been of the type which determines lowering of the tire pressure based on detection values of wheel speed sensors, such as used for an antilock braking system. For example, Japanese Patent Publication No.5-55322 (e.g. claims 1 and 2, FIG. 1) discloses a method for determining lowering of air pressure, for example, based on a ratio between the total of angular velocities of a pair of tires positioned in one diagonal line and the total of angular velocities of the other pair of tires positioned in the other diagonal line. Japanese Patent No.2836652 (e.g. claim 1 and FIG. 4) discloses a tire pressure detecting system for determining lowering of air pressure (underinflation of the tire) based on the fact that the peak value of the tire vibration frequency spectrum lowers or shifts (along the axis of frequency) in accordance with velocity bands of the vehicle and with respect to the normal air pressure values stored in a ROM or the like. These tire pressure monitoring systems perform mechanically stable monitoring of the air pressure.
However, wheel speed sensor calculates wheel speed by means of detecting the number of rotations of the wheel (tire) per unit of time. If the air pressure of the tire changes due to slow leak or the like, the tire diameter per se changes, resulting in a change of the wheel speed. Therefore, the vehicle speed that is calculated from the wheel speed also changes accordingly. The same can be said to a vehicle speed sensor which determines the vehicle speed by means of detecting the rotational speed of the output shaft provided in a transmission. A change in the tire diameter makes the vehicle speed vary. Further, the above tire pressure monitoring system disclosed in Japanese Patent No.2836652 determines the tire pressure per one wheel based on the fact that a change in the air pressure makes the spring constant or the damping characteristic of the side wall of the tire vary so that the peak of the vibration frequency spectrum shifts. Therefore, if the tire is replaced with another one, the aforementioned tire characteristics may vary, which disadvantageously causes an inaccurate determination of the air pressure.
In the tire pressure monitoring system disclosed in Japanese Patent Publication No. 5-55322, it is possible to determine lowering of the air pressure (underinflation of the tire) only for one single wheel or three wheels simultaneously. However, this tire pressure monitoring system, in principle, can not determine the lowering of the air pressure for two or four wheels simultaneously. Even in the case of determining the lowering of the air pressure for one or three wheels simultaneously, the dynamic rolling radius of the tire increases by centrifugal force at vehicle speeds higher than a predetermined value (e.g. 120 km/h). This makes it difficult to determine lowering of the air pressure with sufficient accuracy. On the contrary, the tire pressure monitoring system disclosed in Japanese Patent No.2836652 can determine the lowering of the air pressure simultaneously for four wheels. However, because characteristics of the tire (spring constant or damping characteristic) are different in accordance with tire size, brand of the products, and the like as stated previously, if different kinds of tires (other than recommended products) are attached upon replacement of the tires, it may be difficult or impossible to determine the lowering of the air pressure. Further, even if the aforementioned tire pressure monitoring systems disclosed in Japanese Patent Publication No.5-55322 and Japanese Patent No.2836652 are combined, it is very difficult to simultaneously determine the lowering of the air pressure for four wheels irrespective of vehicle speed or kinds of the tires.
In view of the above, the present invention seeks to provide a tire pressure monitoring system, which is relatively simple in its structure and which can highly accurately monitor air pressure of a tire.